We have investigated the characteristics of 355 nm UV laser diodes. Two laser diode structures were prepared on a grooved AlGaN template as well as on a flat AlGaN template. The threading dislocation densities on the grooved and flat AlGaN were 2.5x108 cm(-2) and 6.5x10(9) cm(-2), respectively. The threshold current density of the laser diode on the grooved AlGaN was 6 kA/cm(2), while no lasing was observed from the laser diode on the flat AlGaN. We also estimated the internal quantum efficiency as well as the injection efficiency in the UV laser diode structures by considering internal quantum efficiency of both optical and electrical excitation. At a carrier density of 1.2x10(19) cm(-3), the internal quantum efficiency of the laser diode structure on the grooved AlGaN was about 50%. This is twice that of the laser diode on the flat AlGaN. At the same time, the internal quantum efficiency estimated from the optical excitation also reached about 50%, even at a carrier density of 3x10(18) cm(-3). This implies that the injection efficiency in our UV laser diode structure was only 25%. Improving the injection efficiency should be effective for significantly reducing the threshold current density. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We fabricated and evaluated GaN/AlGaN multi-quantum wells (MQWs) and ultraviolet laser diodes (UV LDs) on high and low dislocation density underlying layers by epitaxial lateral overgrowth (ELO) method. We analyzed the internal quantum efficiency (IQE) versus carrier concentration characteristics quantitatively by excitation intensity dependent photoluminescence method. The IQE of the MQWs on the ELO AlGaN is 75% when the carrier density is 1x10(19) cm(-3). We demonstrated the UV LD on the ELO AlGaN. However, the UV LD on flat AlGaN did not operate. Also, we investigated the internal loss (alpha(i)) and the IQE multiplied the injection efficiency by changing the reflectivity of the facets. The results showed that the internal loss is 6 cm(-1), and the IQE multiplied by the injection efficiency is 18%. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We fabricated UV laser diodes (LDs). A high-temperature annealing at 850 degrees C for a short duration was found to be very effective for reducing the series resistance of an n-type contact. By applying high-temperature annealing, the operating voltage of an UV LD was reduced from 10.7 V to 9.5 V at a forward current of 300 mA.

We demonstrated activation annealing of Mg-doped p-type Al0.17Ga0.83N in different gases. The hole concentration of Al0.17Ga0.83N annealed in oxygen is higher than that annealed in nitrogen or air. A hole concentration of 1.3 x 10(16) cm(-3) at room temperature was achieved by annealing in oxygen flow at 900 degrees C. Secondary ion mass spectroscopy shows that hydrogen dissociation from Mg-doped Al0.17Ga0.83N is found to be enhanced by annealing in a flow of oxygen, compared with annealing in a flow of nitrogen. We confirmed the effect of activation annealing in oxygen flow on the performance of UV light-emitting diode (LED). At a DC current of 100 mA, the output power of the LED annealed in oxygen flow at 900 degrees C is four times higher than that of the LED annealed in nitrogen flow at 800 degrees C. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Al 0.25 Ga 0.75 N films were grown on a grooved‐Al 0.25 Ga 0.75 N/ AlN/sapphire template by MOVPE. The dislocation density on the grooved areas was as low as 1 × 10 8 cm –2 . We fabricated a UVA light‐emitting diode grown on such an AlGaN underlying layer exhibiting an output power of 12 mW at a DC current of 50 mA with a peak emission wavelength of 345 nm, which corresponds to an external quantum efficiency of 6.7%. This efficiency is the highest reported to date in this wavelength region. We also fabricated a 358 nm UVA laser diode (LD) using a GaN/AlGaN MQW active layer grown on an AlGaN underlying layer. This UV LD exhibits a threshold current of 73 mA and a corresponding current density of 3.8 kA/cm 2 at 7 °C. The characteristic temperature T 0 was 174 K in the temperature range of 7–27 °C. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Annealing in oxygen ambient was found to be effective for realizing a high hole concentration in p-type Al0.17Ga0.83N:Mg. The maximum hole concentration obtained was 1.3 x 10(16) cm(-3) at room temperature. Hydrogen dissociation from Mg-doped Al0.17Ga0.83N is found to be enhanced by annealing in a flow of O-2 compared with annealing in a flow of N-2. (C) 2009 The Japan Society of Applied Physics